Auroras

Dec. 6, 2018: A large hole in the sun’s atmosphere is facing Earth and spewing a stream of solar wind in our direction. NASA’s Solar Dynamics Observatory is monitoring the structure, shown here in a false-color UV image taken on Dec. 6th:

The hole (technical term: “coronal hole”) is so large it almost completely bisects the solar disk, stretching more than a million km across the sun’s equator.

We’ve seen this coronal hole before. It has been spinning around with the sun, lashing Earth with solar wind approximately once a month since September. Last month, the lashing commenced on Nov. 9th, lasted for almost 3 days, and caused sharp tremors in the geomagnetic field. Solar winds blowing faster than 600 km/s sparked an explosion of Phoenix-shaped auroras over Norway:

“The display over Senja, Norway, on Nov. 11th was nothing short of magical,” recalls photographer Adrien Mauduit. “Huge colorful pillars took the shape of a fiery bird.”

The same stream of solar wind will return on Dec. 8th or 9th and it may be even more potent this time because the underlying coronal hole has hrown larger in the intervening month. Arctic sky watchers, mark your calendars and warm your cameras. The Phoenix might rise again. Free:Aurora Alerts.

ZEN ASTRONAUT: Are the holidays stressing you out? Get your zen from the edge of space. On Dec. 2, 2018, the students of Earth to Sky Calculus launched a cosmic ray balloon to the stratosphere. This meditating spaceman pendant went along for the ride:

The students are selling the pendants to support their ballooning program. You can have one for $129.95. They make great gifts for space fans and are guaranteed to soothe holiday stress. Each premium stainless steel pendant comes with a greeting card showing the astronaut in flight and telling the story of its journey to the edge of space and back again.

Oct. 10, 2018: On Oct. 7th, a solar wind stream hit Earth’s magnetic field, sparking a G1-class geomagnetic storm. In southern Finland, the night sky turned green as energetic particles rained down on the upper atmosphere. But there was more to the show than beautiful lights.

“The storm also produced a number of distinctive sounds including crackles and claps,” reports Prof. Emeritus Unto K. Laine of Finland’s Aalto University. “Here is a recording of one of the strongest sounds of the night–a sharp clap.” Click to listen:

“I recorded this in the vicinity of Fiskars village after midnight local time,” he says.

Auroral sounds are controversial. Over the centuries, there have been many reports of strange sounds under the Northern Lights. However, researchers have struggled to explain the phenomenon and sometimes suggested that they might be imaginary. Laine is a believer: “We have been recording sounds like these for almost 20 years as part of the Auroral Acoustics Project.” More samples may be found here.

Laine has developed arrays of microphones that can pinpoint the sounds through triangulation. He finds that they occur about 70 meters above the ground. Temperature inversion layers at that altitude can cause a separation of + and – charges in the air. During some geomagnetic storms, the charge separation breaks down, causing air to move and a faint “clap” to be heard.

Think of it as geomagnetic thunder.

A spectral analysis of the “thunderclap” (above) shows dominant frequencies between 1 kHz and 2 kHz, squarely in the range of human hearing. You have to be quiet to hear them, though.

“People who talk and walk around, concentrating on picture taking, might never hear a single sound related to aurora,” says Laine. “You have to stop all other activities and focus on listening. We Finns are probably good at this because we have received more than 300 reports of sound observations during the Auroral Acoustics Project.”

Over the years, Laine has learned that a geomagnetic storm, by itself, is not enough to produce these thunderclaps. “A strong inversion layer is also required,” he says. “The inversion layer acts like an electrostatic loudspeaker. Without it there are no sounds.” This explains why many geomagnetic storms are silent. The local weather has to be just right — as it was on Oct. 7th.

Sept. 14, 2018: The northern autumnal equinox is only a week away. That means one thing: Cracks are opening in Earth’s magnetic field. Researchers have long known that during weeks around equinoxes fissures form in Earth’s magnetosphere. Solar wind can pour through the gaps to fuel bright displays of Northern Lights. Here’s an example from Yellowknife, Canada:

“On Sept. 5-6, we could see auroras in the sky all night long, with a bright outburst of pink shortly after midnight,” says photographer Yuichi Takasaka.

During the display, a weak stream of solar wind was blowing around Earth. At this time of year, that’s all it takes. Even a gentle gust can breach our planet’s magnetic defenses.

This is called the the “Russell-McPherron effect,” named after the researchers who first explained it. The cracks are opened by the solar wind itself. South-pointing magnetic fields inside the solar wind oppose Earth’s north-pointing magnetic field. North and South partially cancel one another, opening a crack. This cancellation can happen at any time of year, but it happens with greatest effect around the equinoxes. Indeed, a 75-year study shows that September is one of the most geomagnetically active months of the year–a direct result of “equinox cracks.”

NASA and European spacecraft have been detecting these cracks for years. Small ones are about the size of California, and many are wider than the entire planet. There’s no danger to people on Earth. Our planet’s atmosphere intercepts the rush of incoming particles with no harm done and a beautiful afterglow.

Sept. 2, 2018: Picture this: A billion-ton coronal mass ejection (CME) slams into Earth’s magnetic field. Campers in the Rocky Mountains wake up in the middle of the night, thinking that the glow they see is sunrise. No, it’s the Northern Lights. People in Cuba read their morning paper by the red illumination of aurora borealis. Earth is peppered by particles so energetic, they alter the chemistry of polar ice.

Hard to believe? It really happened 159 years ago. This map shows where auroras were sighted in the early hours of Sept. 2, 1859:

As the day unfolded, the gathering storm electrified telegraph lines, shocking technicians and setting their telegraph papers on fire. The “Victorian Internet” was knocked offline. Magnetometers around the world recorded strong disturbances in the planetary magnetic field for more than a week.

The cause of all this was an extraordinary solar flare witnessed the day before by British astronomer Richard Carrington. His sighting on Sept. 1, 1859, marked the discovery of solar flares and foreshadowed a new field of study: space weather. According to a NASA-funded study by the National Academy of Sciences, if a similar “Carrington Event” occurred today, it could cause substantial damage to society’s high-tech infrastructure and require years for complete recovery.

In Sept. 1859, this large sunspot unleashed a record-setting solar flare. Sketch by R. C. Carrington.

Could it happen again? In fact, a similar event did happen only 6 years ago. On July 23, 2012, a powerful explosion on the sun hurled a Carrington-class CME away from the sun. Fortunately, it missed. “If it had hit, we would still be picking up the pieces,” says Prof. Daniel Baker of the University of Colorado, who summarized the event at NOAA’s Space Weather Workshop in 2014.

In a paper published just a few months ago, researchers from the University of Birmingham used Extreme Value Theory to estimate the average time between “Carrington-like flares.” Their best answer: ~100 years, a value which suggests we may be overdue for a really big storm.

Forecasters did not see this coming. The stage was set for the storm when a minor CME arrived with little fanfare about 24 hours ago. First contact with the CME barely registered in solar wind data, and Earth’s magnetic field was unperturbed. The action began only after Earth entered the CME’s wake, where strong south-pointing magnetic fields opened a crack in our planet’s magnetosphere. A surprise geomagnetic storm ensued.

April 20, 2018: An interplanetary shock wave hit Earth’s magnetic field on April 19th around 23:50 UT. When the disturbance arrived, the density of solar wind flowing around our planet abruptly quadrupled and a crack opened in Earth’s magnetic field. The resulting G2-class geomagnetic storm sparked unusual “electric blue” auroras.

“I’ve been flying airplanes for 20 years and photographing aurora for 10 years, but I’ve never seen anything like this before,” reports pilot Matt Melnyk who photographed the display from 39,000 feet:

“Electric blue auroras!” he says. “This was while on a red eye flight from Edmonton to Toronto around 4 am over northern Manitoba. Unbelievable sky. I was able to grab some hasty shots with a cell phone.”

Auroras are usually green–a sign of oxygen. Rare blue auroras are caused by nitrogen molecules. Energetic particles striking N2+ at the upper limits of Earth’s atmosphere can produce an azure glow during intense geomagnetic storms.

During the storm, Northern Lights spilled across the Canadian border into the United States as far south as Indiana. Hongming Zheng, a student at Purdue University, saw the blue glow just five miles from his dorm:

“I was preparing for bed at 1:32 am on April 20th when I read that there was an Interplanetary Shockwave,” says Zheng. “I immediately started driving north to see the show. A weak green wisp showed up at 2am and faded, but shortly after 5am a sudden outburst occurred. Purple pillars were easily visible to the naked eye. It’s funny how one minute you are in a humid dorm struggling to get the laundry door closed, and the next minute you are chasing one of the most spectacular phenomenon known to man.”

What is an interplanetary shock wave? It is a supersonic disturbance in the gaseous material of the solar wind. These waves are usually delivered by coronal mass ejections (CMEs). Indeed, this one might have been a minor CME that left the sun unrecognized earlier this week.

Alternately, it might have been an unusually sharp co-rotating interaction region (CIR). CIRs are transition zones between slow- and fast-moving streams of solar wind. They contain plasma density gradients and magnetic fields that often do a good job sparking auroras.

April 11, 2018: Yesterday, a G1-class geomagnetic storm was brewing over Canada as a stream of solar wind buffeted Earth’s magnetic field. Matthew Wheeler of Robson Valley, British Columbia, stepped outside to see what was up–and STEVE appeared. “My dog barked at it for the entire hour it was visible,” says Wheeler. “It was flowing like a river at astonishing speed.” Click to play his must-see video:

STEVE may look like an aurora, but it is not. For one thing, it is soft purple, not green like typical auroras. And it has its own special form–tightly collimated into a narrow ribbon that can bisect the entire sky.

Researchers are only beginning to understand the phenomenon–aided by a chance encounter between STEVE and a European satellite a few years ago. In situ measurements revealed that STEVE is a hot (3000 degrees C) ribbon of ionized gas slicing through Earth’s upper atmosphere some 300 km above the ground. It appears unpredictably during some, but not all, geomagnetic storms.

Another video–“my best yet,” says Wheeler–shows the beautiful interaction between the soft-purple ribbon and nearby green “picket fence” auroras:

“The purple ribbon was moving much faster than the green pickets,” says Wheeler. “And while their forms varied from smooth to ragged and back again, their path across the sky was almost constant for the whole hour–as it has since I first noticed STEVE over this valley in the 1980s.”

Does STEVE really make dogs bark? “Mine does,” says Wheeler. “In addition to barking at STEVE, my giant Akbash astronomy dog, Patch, has barked at the space station since he was a pup, and proudly seen it off the farm every time. He is also a valuable spotter of meteor showers. When I hear him barking upwards, it is time to go outside.”

March 25, 2018: Last night, something happened at the edge of space over Alaska. More than 200 km above Anchorage, a hot ribbon of ionized gas sliced through Earth’s magnetosphere, creating a luminous arc that rivaled the Moon in brightness. Sanjana Greenhill witnessed the apparition:

“We noticed this perfect arc developing across the sky,” says Greenhill. “It didn’t seem like the aurora since it wasn’t moving much. The arc got brighter and then faded and then got brighter again. And then it dawned on me, this is STEVE!”

STEVE is an aurora-like phenomenon that researchers are only beginning to understand. For many years, northern sky watchers reported the form occasionally dancing alongside auroras. It was widely called a “proton arc” until researchers pointed out that protons had nothing to do with it. So members of the Alberta Aurora Chasers group gave it a new name: “Steve” (since upgraded to STEVE, an acronym for ‘Strong Thermal Emission Velocity Enhancement’).

The first clues to the nature of STEVE came in 2016 when one of the European Space Agency’s Swarm satellites encountered the phenomenon. “As the satellite flew straight though ‘Steve,’ the temperature jumped by 3000°C and the data revealed a 25 km-wide ribbon of gas flowing westwards at about 6 km/s (13,000 mph),” reports Eric Donovan from the University of Calgary.

Donovan and a team of colleagues led by Elizabeth MacDonald of NASA’s Goddard Space Flight Center have just published a paper on STEVE. In it, they confirm that STEVE is distinct from ordinary auroras, usually forming to the south of active Northern Lights. The mauve and purple colored arcs, they say, are related to supersonic rivers of gas called “subauroral ion drifts” (SAIDs), which flow through Earth’s magnetic field. Earth-orbiting satelites have tracked thousands of SAIDs: they tend to appear near latitude +60 degrees, and occur more frequently during spring and fall than summer and winter.

This last point means that now is the season for STEVE. The onset of northern spring seems to lure the arc out of winter hiding.

“I saw STEVE for the first time on March 18th,” reports Giuseppe Petricca , who took this sequence of pictures from the Isle of Lewis in Scotland:

“It was an ever-changing tornado, with violet tones, always in movement, always with different shapes,” he says. “Another wonder of Nature!”

The mystery of STEVE is far from solved. Researchers still don’t understand why STEVE is purple–or for that matter why the underlying rivers of gas should glow at all. “Further spectral analysis and modeling are needed,” say MacDonald et al.

March 11. 2018: The vernal equinox is less than 10 days away. That means one thing: Cracks are opening in Earth’s magnetic field. Researchers have long known that during weeks around equinoxes fissures form in Earth’s magnetosphere. Solar wind can pour through the gaps to fuel bright displays of Arctic lights. One such episode occurred on March 9th. “The sky exploded with auroras,” reports Kristin Berg, who sends this picture from Tromsø, Norway:

During the display, a stream of solar wind was barely grazing Earth’s magnetic field. At this time of year, that’s all it takes. Even a gentle gust of solar wind can breach our planet’s magnetic defenses.

This is called the the “Russell-McPherron effect,” named after the researchers who first explained it. The cracks are opened by the solar wind itself. South-pointing magnetic fields inside the solar wind oppose Earth’s north-pointing magnetic field. The two, N vs. S, partially cancel one another, weakening our planet’s magnetic defenses. This cancellation can happen at any time of year, but it happens with greatest effect around the equinoxes. Indeed, a 75-year study shows that March is the most geomagnetically active month of the year, followed closely by September-October–a direct result of “equinox cracks.”

NASA and European spacecraft have been detecting these cracks for years. Small ones are about the size of California, and many are wider than the entire planet. While the cracks are open, magnetic fields on Earth are connected to those on the sun. Theoretically, it would be possible to pick a magnetic field line on terra firma and follow it all the way back to the solar surface. There’s no danger to people on Earth, however, because our atmosphere protects us, intercepting the rain of particles. The afterglow of this shielding action is called the “aurora borealis.”

Nov. 23, 2017: On Nov. 22nd, the face of the sun was unblemished by sunspots, and NOAA classified solar activity as “very low.” Nevertheless, the skies above Tromsø, Norway, exploded with a remarkable outburst of pink auroras. “Suddenly, the whole valley turned white (with a hint of pink),” says Frank Meissner, who witnessed and photographed the display. “It was over after about 20 seconds.”

How bright was it? “The brightness of the auroras may be compared to the car lights in the background of my photo,” points out Meissner.

In nearby Kvaløya, aurora tour guide Marianne Bergli witnessed a surge of pink that was, if anything, even more dramatic:

This outburst was powered by a stream of solar wind flowing from a hole in the sun’s atmosphere. Such holes are common during Solar Minimum, and they require no sunspots to form. That’s why auroras continue throughout the 11-year solar cycle.

The pink color of the outburst tells us something interesting about the solar wind on Nov. 22nd: it seems to have been unusually penetrating. Most auroras are green–a verdant glow caused by energetic particles from space hitting oxygen atoms 100 km to 300 km above Earth’s surface. Pink appears when the energetic particles descend lower than usual, striking nitrogen molecules at the 100 km level and below.

In recent winters, big displays of pink and white auroras have coincided with spotless suns often enough to make observers wonder if there is a connection. If so, more outbursts are in the offing as the sun continues its plunge toward a deep Solar Minimum. Stay tuned for pink!